At 8 weeks of pregnancy, the developing baby measures roughly 16 to 20 millimeters from head to bottom, about the size of a kidney bean. It weighs approximately 1 gram, or 0.04 ounces. Despite being tiny enough to sit on your fingertip, this is a period of rapid and dramatic development.
Crown-to-Rump Measurements at 8 Weeks
Doctors measure early pregnancies using “crown-rump length,” which is the distance from the top of the head to the bottom of the torso (legs aren’t included because they’re curled up tight). At 8 weeks, that measurement falls between about 16 and 20 millimeters, or roughly 0.6 to 0.8 inches, depending on the exact day. The measurement changes noticeably even within a single week: at 8 weeks and 1 day the average is 16 mm, while by 8 weeks and 4 days it’s already 20 mm. That rapid growth is why ultrasound technicians use crown-rump length to confirm or adjust your due date during the first trimester.
How Weight Compares
One gram is almost nothing in practical terms. For perspective, a single paperclip weighs about the same. Most of the baby’s volume at this stage is water and developing tissue rather than bone or fat, which come later. Weight gain accelerates significantly in the weeks ahead, but at 8 weeks, size is tracked almost entirely by length rather than weight.
What’s Developing at This Stage
Eight weeks marks a turning point. By the end of this week, all of the major organs and body systems have started to take shape. The heart has been pulsing since around weeks 5 to 6, and the neural tube that becomes the brain and spinal cord formed even earlier. Arms and legs started as tiny buds around week 6 and now look more like small paddles, with webbed fingers and toes just beginning to separate.
Eyes are becoming visible and ears are starting to form. Internally, the foundations of the digestive system, lungs, and kidneys are all in progress. None of these organs are functional yet in the way they’ll need to be at birth, but the basic architecture is being laid down right now. This is why the first trimester is considered such a sensitive period for development.
Embryo or Fetus?
Technically, at 8 weeks of pregnancy your baby is still called an embryo. The American College of Obstetricians and Gynecologists defines the embryonic period as lasting 8 weeks after fertilization. Starting at week 9, the term switches to “fetus,” which it remains until birth. The distinction isn’t just vocabulary. The embryonic stage is when structures form from scratch, while the fetal stage is when those structures grow and mature. So 8 weeks sits right at the boundary between building the blueprint and filling it in.
What You’d See on an Ultrasound
If you have an ultrasound around 8 weeks, it’s typically done transvaginally because the baby is still too small to image well through the abdomen. On screen, you should see a recognizable head and body, though they won’t look proportional the way you’d expect. The head is oversized compared to the rest, which is normal. The beginnings of limb buds are usually visible, and the heartbeat is easy to detect. Some practitioners can even spot subtle body movements at this stage, though you won’t feel any movement yourself for several more weeks.
The yolk sac, a small circular structure that provides early nutrition before the placenta takes over, is also typically visible next to the embryo. Don’t be surprised if the image looks more like a blob with a flickering heartbeat than anything resembling a baby. That’s completely normal at this size.
Changes in Your Body
While the baby is still smaller than a grape, your body is already adapting. By week 8, your uterus has grown from its usual size (roughly the size of a small pear) to about the size of a tennis ball. That growth isn’t visible from the outside yet for most people, but you may notice your pants fitting a bit differently, along with bloating and breast tenderness. The hormonal changes driving all of this development are also responsible for common 8-week symptoms like nausea, fatigue, and heightened sense of smell.